1. Field of the Invention
The present invention relates to a light scanning apparatus used in devices such as a barcode reader, a retina projection display, a laser printer, and a laser projector, which read still images and moving images by light scanning or which output still images and moving images by light scanning.
2. Description of the Related Art
For instance, in a laser projector, a predetermined vision is projected on a screen by irradiating a scanning mirror with a laser light and scanning the screen with the light reflected by the scanning mirror. In order to project the predetermined vision on the screen, a posture of the scanning mirror must be controlled. Since the scanning mirror is controlled on the basis of posture information indicating the posture of the scanning mirror, a present posture of the scanning mirror must be firstly detected in order to control the posture of the scanning mirror.
Further, in a light scanning apparatus which may cause light to enter human's eyes, irradiation of the light needs to be stopped immediately after light scanning is finished. On the basis of the posture information indicating the posture of the scanning mirror, it is possible to determine whether light scanning is being performed by the scanning mirror. Accordingly, in order to stop the irradiation of the light immediately after the light scanning is finished, the present posture of the scanning mirror must be firstly detected.
Furthermore, in the case where a vision is directly projected on a retina of eyeball by use of the light scanning apparatus, the retina may be damaged when the vision continues to be projected on a predetermined portion on the retina over a length of safety standard time while retaining the scanning mirror at a fixed posture. Accordingly, by measuring a time that the scanning mirror is retained at the fixed posture, the projection of the vision onto the retina must be stopped when the length of time that the scanning mirror at the fixed posture reaches the safety standard time. The length of time that the scanning mirror is retained at the fixed posture can be measured by detecting the posture of the scanning mirror. Consequently, the present posture of the scanning mirror must be detected.
FIG. 26 is a plan view showing a first related art posture detecting device 1 for detecting a posture of an object. FIG. 27 is a plan view showing an enlarged region II of FIG. 26.
In the first related art, posture information indicating the posture of the object is obtained by use of a property of silicon single crystal that electrical resistance thereof changes. The posture detecting device 1 comprises a movable portion 2, a support portion 3, and first connection portion 4A and second connecting portion 4B for coupling the movable portion 2 to the support portion 3. The movable portion 2 has a schematic rectangular parallelepiped shape formed of a platy body. The first connecting portion 4A and the second connecting portion 4B protrude in a direction away from a middle portion in a longitudinal direction of the movable portion 2. The movable portion 2 is connected to the support portion 3 by the first connecting portion 4A and second connecting portion 4B so that the movable portion 2 is angularly displaceable about an axial line which is perpendicular to thickness direction and longitudinal direction of the movable portion 2.
The first connection portion 4A is formed of silicon single crystal. The electrical resistance of the silicon single crystal changes by piezoresistance effect depending on stress which is added thereto. When the movable portion 2 is angularly displaced about the above-mentioned axial line, the stress is added to the first connecting portion 4A so that the electrical resistance of the first connection portion 4A formed of silicon single crystal changes by piezoresistance effect. By measuring this change amount of electrical resistance, it is possible to measure an amount of angular displacement of the movable portion 2.
In order to measure the change of the electrical resistance of the first connecting portion 4A, a four-terminal method is used. On one surface portion of the first connection portion 4A are formed a first electrode terminal 5A, a second electrode terminal 5B, a third electrode terminal 5C, and a four electrode terminal 5D. The first electrode terminal 5A and the second electrode terminal 5B are connected at a predetermined distance to parallel electrodes 6 which are electrically connected to a surface of the first connecting portion 4A while the third electrode terminal 5C and the fourth electrode terminal 5D are connected, between the parallel electrodes 6, at a predetermined distance to measuring electrodes which are electrically connected to the one surface of the first connecting portion 4A. By flowing constant electric current between the parallel electrodes via the first electrode terminal 5A and the second electrode terminal 5B so as to measure a voltage generated between the third electrode terminal 5C and the fourth electrode terminal 5D, it is possible to measure the change of the electrical resistance of the first connecting portion 4A (refer to, for instance, U.S. Pat. No. 5,648,618, Japanese Unexamined Patent Publication JP-A 2002-524271, and “Semiconducting Stress Transducers utilizing the Transverse and Shear Piezo Resistance Effects” written by W. G. Pfann and R. N. Thurston, on page 2008 in the Journal of Applied Physics, Vol. 32, No. 11 (1961)).
In a second related art, by use of induced electromotive force, a posture of a movable member capable of being angularly displaced about a predetermined axial line is detected. On the movable member is formed a loop-shaped speed detecting coil. Moreover, the movable member is disposed in a magnetostatic field. When the movable member is angularly displaced, a magnetic flux passing through a region surround by the speed detecting coil of the movable member changes so that the induced electromotive force is generated. By measuring a magnitude of this induced electromotive force, it is possible to measure an amount of angular displacement of the movable member (refer to, for instance, Japanese Unexamined Patent Publication JP-A 11-288444 (1999)).
In the first related art, the amount of angular displacement of the movable portion 2 is detected by measuring the electrical resistance of the first connecting portion 4A which changes depending on the to-be-added stress. However, the change amount of electrical resistance is subtle, and dependent on temperatures. The subtle change amount of electrical resistance is measured by use of the four-terminal method. In order to measure the resistance by use of the four-terminal method, a source of constant electric current is necessary. This leads the necessity of forming a constant electric current circuit. As a result, a problem arises that a circuit for measuring the amount of angular displacement becomes complex. In addition, since the change amount of electrical resistance is subtle, an amplifying circuit for amplifying detected electrical resistance information becomes necessary, which causes a problem that the circuit becomes more complex. Furthermore, in a case where the movable portion 2 is irradiated with a light and then displaced, thereby performing scanning with a light reflected by the movable portion 2, a temperature of the movable portion 2 increases by the emitted light so that a temperature of the first connection portion 4A in contact with the movable portion 2 also increases. Since the change amount of electrical resistance of the first connecting portion 4A is dependent on temperatures, a problem arises that the amount of angular displacement of the movable portion 2 cannot be correctly measured. Particularly, in a case where a diameter of the first connecting portion 4A is as small as several tens of μm, heat generated on the movable portion 2 is more hardly transferred to the support portion 3 via the first connecting portion 4A so that the temperature of the movable portion 2 further increases. Accordingly, the temperature of the first connecting portion 4A further increases, which causes a problem that the amount of angular displacement of the movable portion 2 cannot be correctly measured. In addition, the temperature of the first connecting portion 4A changes also by an atmosphere temperature of the first connecting portion 4A, which causes a problem that the amount of angular displacement of the movable portion 2 cannot be correctly measured.
In the second related art, the amount of angular displacement of the movable member is measured by measuring the magnitude of the induced electromotive force which is generated by the angular displacement of the movable member. In a case a light scanning apparatus comprising this movable member is disposed for use in a predetermined magnetic field, a magnetic field where the movable member is disposed changes. When the magnetic field having the movable member disposed therein changes, the magnetic flux in the region which is surrounded by the loop-shaped speed detecting coil formed on the movable member changes. Accordingly, the magnitude of the induced electromotive force generated by the angular displacement of the movable member also changes, which causes a problem that the amount of angular displacement of the movable member cannot be correctively detected. For instance, in a case where the light scanning apparatus is disposed in such a magnetic field that the magnetic flux in the region which is surrounded by the loop-shaped speed detecting coil formed on the movable member becomes zero, the induced electromotive force is not generated, even when the movable member is angularly displaced, so that the amount of angular displacement of the movable member cannot be detected.